**Core Concept**
Osmotic fragility is a measure of the resistance of red blood cells (RBCs) to hemolysis when exposed to varying concentrations of saline solution. Normally, RBCs can withstand a certain degree of osmotic stress without lysing. However, an abnormal decrease in osmotic fragility can lead to premature hemolysis due to excessive water influx into the cells.

**Why the Correct Answer is Right**
In conditions where the RBCs have a decreased osmotic fragility, they are more susceptible to hemolysis when exposed to hypotonic solutions. This occurs due to the loss of membrane surface area and the resulting decrease in the number of spectrin molecules, which normally maintain the cell's shape and integrity. As a result, the RBCs are unable to withstand the osmotic stress and undergo hemolysis.

**Why Each Wrong Option is Incorrect**
**Option A:** This is a distractor, as the question is asking for a condition that decreases osmotic fragility, not increases it.
**Option B:** This is not a condition that decreases osmotic fragility; rather, it is a condition that increases it.
**Option C:** This is a distractor, as it is a condition that affects RBCs, but it does not decrease osmotic fragility.

**Clinical Pearl / High-Yield Fact**
Decreased osmotic fragility is often seen in conditions where there is a loss of membrane surface area, such as in hereditary spherocytosis. This is due to the decrease in spectrin molecules, which normally maintain the cell's shape and integrity.

**Correct Answer:** C. Hereditary spherocytosis.